This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-383881, filed Dec. 18, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a rotary-anode-type X-ray tube, and more specifically, to a rotary-anode-type X-ray tube in which an anode target is rotatably supported by means of a rotary mechanism having dynamic-pressure slide bearings.
2. Description of the Related Art
A rotary-anode-type X-ray tube is designed so that electron beams are applied to an anode target that rotates at high speed and X-rays are emitted from the anode target. Usually, in the X-ray tube constructed in this manner, the anode target is rotatably supported by means of a rotary mechanism in which bearings are arranged between a rotary cylinder and a stationary shaft.
A conventional rotary-anode-type X-ray tube will now be described with reference to FIGS. 1A to 1D. In FIGS. 1A to 1D, numeral 31 denotes an anode target that emits X-rays. The target 31 is coupled to a rotary mechanism 33 by means of a rotating shaft 32.
The rotary mechanism 33 comprises a rotary structure and a stationary structure. The rotary structure is composed of a rotary cylinder 34 in the form of a bottomed cylinder, as shown in FIG. 1A. As shown in FIGS. 1A and 1B, the stationary structure is composed of a substantially columnar stationary shaft 35 that is fitted in the rotary cylinder 34. The bottom opening of the rotary cylinder 34 is sealed liquid-tight by means of a closer 36.
Pair of herringbone-pattern helical grooves 37A and 37B are formed individually in two positions, top and bottom, on the outer peripheral surface of the stationary shaft 35. A liquid-metal lubricant is fed into the helical grooves 37A and 37B and bearing gaps in which the stationary shaft 35 and the rotary cylinder 34 face each other. The helical grooves and the bearing gaps constitute radial dynamic-pressure slide bearings 38 and 39, individually.
A small-diameter region 351 that has an outside diameter smaller than those of the regions for the dynamic-pressure slide bearings 38 and 39 is formed in a part of the stationary shaft 35, e.g., in that portion which is located between the upper and lower helical grooves 37A and 37B. An annular space 40 is defined between the small-diameter region 351 of the stationary shaft 35 and the rotary cylinder 34. The space 40 serves as a storage chamber that stores the liquid-metal lubricant.
As shown in FIG. 1B, herringbone-pattern helical grooves 41 are formed in a circle on the upper end face of the stationary shaft 35 and the upper surface of the closer 36, individually. The liquid-metal lubricant is fed into the helical grooves 41, a bearing gap in which the upper end face of the stationary shaft 35 and the base of the rotary cylinder 34 face each other, a bearing gap in which the upper surface of the closer 36 and a lower step portion of the stationary shaft 35 face each other, etc. The helical grooves and the bearing gaps constitute thrust dynamic-pressure slide bearings 42 and 43.
As shown in the sectional view of FIG. 1C taken along line ICxe2x80x94IC of FIG. 1A, the central portion of the stationary shaft 35 is provided with a reservoir 44 that extends along a tube axis and serve to store the liquid-metal lubricant. FIG. 1D is a sectional view of the stationary shaft 35 shown in FIG. 1A, taken along line ID-O-ID of FIG. 1C. In three positions 10A, 10B and 10C that are spaced along the axis of the reservoir 44, as shown in FIGS. 1C and 1D, three sets of ducts 45A, 45B and 45C that radially diverge extend at equal angular spaces of 120 degrees in the circumferential direction. The ducts 45A that are situated in the upper part of FIG. 1A and the ducts 45C that are situated in the lower part of FIG. 1A open into the helical grooves 37A and 37B that constitute the dynamic-pressure slide bearings 38 and 39, respectively, while the ducts 45B that are situated in the middle part of FIG. 1A opens into the small-diameter region 351 of the stationary shaft 35.
When the X-ray tube is actuated to cause the rotary structure of the rotary mechanism to rotate, the liquid-metal lubricant in the reservoir 44 circulates through the ducts 45A, 45B and 45C, helical grooves of the dynamic-pressure slide bearings 38, 39, 42 and 43, bearing gaps, etc. Thus, the bearing portions can be prevented from being exhausted of the lubricant. The reservoir 44 serves not only as a passage through which the lubricant circulates but also as a passage through which gases produced in the bearings are circulated.
If any gas pools are formed in the reservoir through which the liquid-metal lubricant circulates, during the operation of the conventional rotary-anode-type X-ray tube, the lubricant may fail to circulate satisfactorily, so that the bearing portions may be exhausted of the lubricant, in some cases. In consequence, the rotation of the rotary structure that constitutes the rotary mechanism becomes unstable. In the worst case, the so-called cling occurs such that a part of the rotary cylinder directly touches a part of the stationary shaft, whereby the rotation of the rotary cylinder is stopped.
If a plurality of ducts are provided diverging from the reservoir, the respective distal ends of some ducts, such as those ones which are situated at the top and bottom, open in the helical groove portions on the outer periphery of the stationary shaft. Accordingly, the respective positions of the end openings of the ducts must be aligned individually with those of the helical grooves. Thus, the manufacture is difficult, requiring high mechanical accuracy.
The object of the present invention is to provide a rotary-anode-type X-ray tube, which can be manufactured with ease and in which a rotary structure that constitutes a rotary mechanism can rotate satisfactorily.
According to the present invention, there is provided a rotary-anode-type X-ray tube, which comprises: an anode target;
a rotary cylinder coupled mechanically to the anode target and having an inner surface inside;
a columnar stationary shaft having a central axis, opposite end faces, a pair of large-diameter portions, and a small-diameter portion between the large-diameter portions, the stationary shaft being fitted in the rotary cylinder, the large- and small-diameter portions having an outer surface each, the outer surface of the small-diameter portion and the inner surface of the rotary cylinder defining an annular first reservoir, the stationary shaft having second reservoirs extending along the central axis therein and a plurality of groups of ducts, wherein each of the second reservoir is connected to the first reservoir by the ducts of the group;
radial dynamic-pressure slide bearings located between the respective outer surfaces of the large-diameter portions and the inner surface of the rotary cylinder, individually;
a thrust dynamic-pressure slide bearing provided between an end face of the stationary shaft and the inner surface of the rotary cylinder; and
a liquid-metal lubricant filling the first and second reservoirs, ducts, and radial and thrust dynamic-pressure slide bearings.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.